Preparation of conjugated polyphenylenes from maleimide-based enediynes through thermal-triggered Bergman cyclization polymerization
Several maleimide-based enediynes are synthesized through Sonogashira coupling reactions between 3,4-diiodo-N-benzylmaleimide (3) and para-substituted phenylacetylenes (5). All these enediynes undergo thermal-triggered Bergman cyclization at elevated temperature (130–160° C) as revealed by DSC analysis. Conjugated polyphenylenes (7) are obtained by simply heating these enediynes (6) under vacuum. The occurrence of Bergman cyclization is evidenced by IR and NMR spectroscopy. The conjugated polyphenylenes are further characterized by UV-vis and MALDI-TOF mass spectroscopy (MS). Isotope pattern analysis on the MS spectra of the conjugated polyphenylenes shows that the chain ends of the polymers consist of a considerable amount of unmasked free radicals. EPR spectroscopy of the conjugated polyphenylenes confirms the existence of free radicals, which slowly fade away probably by removing active atoms (like hydrogen or halogen) from the environment.